hwmon: qpnp-adc-common: Clamp voltage value

On VADC_HC peripheral the ADC code can extend beyond
0-0x4000 range if the input is below 0V or if the
offset and gain error needs to be calibrated out.
Clamp the values for cases where the code represents
the value below 0V.

Change-Id: I2ede3d86d812fbdf757395d92e08e70636e9965a
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>